Commercial griddles have a number of special requirements that distinguish them from other cooking units. A large flat cooking surface is required, capable of being scraped clean after frying of food; the cooking surface must be capable of delivering a high density heat flux variously over its area at a well-controlled temperature as high as 400.degree. F. (204.degree. C.); and the surface must retain its flatness over the wide range of thermal stresses that such variable loading implies. Desirable beyond this is a griddle that has no unevenly hot areas and one that is efficient in use of energy.
Griddles available today are constructed, as they have been for many decades, in the form of an extremely thick and heavy flat plate, heated from below by gas burners or electric elements. These require a long warm-up time and give uneven heating over the griddle surface. For these and other reasons they require an experienced operator and are wasteful of energy. Further, their uneven heating makes impracticable cooking "by the clock" a single product (e.g., hamburgers) over the entire griddle surface, as is strongly desired in fast-food restaurants.
It has been known for a long time that cooking surfaces can in principle be heated by vapor transfer techniques to achieve a uniform temperature. Examples are Swiss Pat. No. 258,804; French Pat. No. 529,997; and U.S. Pat. Nos. 2,279,000; 2,434,016; 2,595,527 and 3,823,307. But these proposals have not contributed an effective griddle.
With the advent of the heat pipe technology has come an attempt to apply the specialized heat pipe mode of vapor transfer heating to griddles using a localized heat source to which the transfer liquid is wicked by suitable wicking members. Examples are U.S. Pat. Nos. 3,853,112 and 3,968,787 and see also U.S. Pat. No. 3,603,767 and 3,791,372. These have failed to satisfactorily solve the complex set of problems that the high performance griddle imposes, as a result have not gone into commercial use, and in any event have serious drawbacks.